In recent times, modern motor vehicles have often been equipped with level control systems of the kind described above. These level control systems afford the advantage that the spacing of the vehicle chassis relative to a fixed reference point can be held constant independently of the state of loading of the vehicle. However, it is noted that modern level control systems are also burdened with a safety risk. For example, the situation can occur that a vehicle, which is equipped with a level control system, is heavily loaded. In this situation, the air pressure in the air springs must be increased by filling the latter in order to maintain the spacing of the vehicle chassis relative to the reference point constant. Correspondingly, after unloading the motor vehicle, the air pressure in the air spring must again be reduced by discharging the air spring. The filling and discharging of the air spring is controlled by the above-mentioned control unit in that this control unit transfers the switchable valves into a corresponding switching state. If a fault occurs in the first control unit during the loaded condition of the motor vehicle, then a discharge of the air springs is no longer possible, so that, after unloading the motor vehicle, the air pressure in the air springs can no longer be reduced. This has the consequence that the vehicle chassis is lifted relative to the reference point after the vehicle is unloaded. This can lead to the condition that the center of gravity of the motor vehicle after unloading lies very high so that the motor vehicle is in a dynamically unsafe driving condition.
A further safety risk can occur in an off-road vehicle equipped with a level control system. Accordingly, and to provide a large ground clearance of the off-road vehicle, the center of gravity of the vehicle can be brought to a very high level with the aid of the level control system. This takes place by filling the air springs with pressurized air. If the ground clearance of the off-road vehicle is no longer needed, then the air springs must be vented. The filling operation as well as the discharging operation are controlled by the above-mentioned control unit which transfers the switchable valves into a corresponding switching condition. A situation can occur wherein the off-road vehicle has a high ground clearance and a fault occurs in the first control unit which leads to a situation that the air springs can no longer be discharged. In such a situation, dropping the chassis of the off-road vehicle is no longer possible. The off-road vehicle is then in a dynamically unsafe driving condition because of the high center of gravity.